THIS INVENTION relates to the recalibration of sensors and instruments used for chemical analysis.
Flow based methods of analysis require periodic recalibration with standards. Generally, several external standards (typically between two to five depending on the characteristics of the sensor or instrument e.g. linear or non-linear) are measured at intervals between a group of samples, but in all periodic recalibrations, a compromise exists. Very frequent recalibration improves reliability, but at the expense of sample throughput. Infrequent recalibration increases the risk of an out of tolerance condition between recalibrations, requiring the sample measurements to be reworked.
U.S. Pat. No. 5,080,866 describes a cyclic flow based analytical technique (Discontinuous Flow Analysis) where the ratio of a reagent to sample is varied during the measurement half of the cycle, and one example of the technique shows how an internal recalibration can be performed with a single standard and stepped flow profile on every sample. This overcomes the problem of the frequency of recalibration inherent in the use of external standards. However, the concentration of the internal standard relative to the sample has to be within certain limits so that the accuracy of the recalibration is not impaired. If the sample and standard are close in concentration, the step changes are small, consequently the resolution may be insufficient for an accurate recalibration. If the sample and standard are considerably different in concentration, plateau responses for each step may not be achieved due to the detector response time, or diffusion effects.
It should be noted that in all cyclic analytical techniques which use piston pumps, the cycle for each pump comprises two complementary half-cycles: a forward stroke whereby fluid is expelled from the pump, and a return stroke whereby fluid is drawn into the pump. The measurement of the sample takes place during one of the half-cycles.